1. Field of the Invention
The present invention relates to an improvement on a gate drive circuit for on-and-off-driving an insulated gate semiconductor device such as a power MOSFET, an IGBT (insulated gate bipolar transistor) and a MOSGTO and to a flash controller for photography etc. using the improved gate drive circuit.
2. Description of the Background Art
The type of gate drive circuit which has been conventionally used is shown in FIG. 9. Hereinafter described is the structure of the conventional gate drive circuit based on the example of FIG. 9. In FIG. 9, numeral 100 designates a gate drive circuit for receiving a control input V.sub.1, which comprises two switching transistors 101 and 102 connected in series between a gate drive power source V.sub.GG and the ground, a gate resistor 110 connected between an output terminal and a midpoint of the connection between the two switching transistors 101 and 102, and logical circuits 103 and 104 for controlling the two switching transistors 101 and 102 by the control input V.sub.1. An output from the gate drive circuit 100 is connected to a portion between gate and source of a power MOSFET 3 which is one type of insulated gate semiconductor devices. The power MOSFET 3 constitutes a main circuit in cooperation with a load 4 and a power source 5.
FIG. 10 shows operating waveforms of the gate drive circuit 100 of FIG. 9. Referring to this waveform chart, an operation of the conventional gate drive circuit is explained hereinafter. Responsive to the control input V.sub.1, the logical circuits 103 and 104 turn on/off the switching transistors 101 and 102 complementarily, respectively. When the control input V.sub.1 is low, the switching transistor 101 is off and the switching transistor 102 is on. Accordingly, a midpoint voltage v.sub.2 of the two switching transistors 101 and 102 is low (V.sub.2L), and a gate voltage v.sub.G of the power MOSFET 3 is constrained at a low level (V.sub.GL) which is substantially equal to the ground level. When the control input V.sub.1 is high, the switching transistor 101 is on and the switching transistor 102 is off. Accordingly, the midpoint voltage v.sub.2 of the two switching transistors 101 and 102 is a high level (V.sub.2H) which is substantially equal to V.sub.GG. By virtue of this high level, a gate current i.sub.G is supplied through the gate resistor 110 to the gate of the power MOSFET 3.
As for the power MOSFET 3, the gate thereof has capacitance characteristics, as is well-known. When a gate-source capacitance C.sub.GS and a gate-drain capacitance C.sub.GD are charged, the gate voltage v.sub.G is increased. The capacitance C.sub.GD of the power MOSFET 3 varies widely according to the voltages of gate and drain and carries out considerably complicated charging and discharging operations, partially caused by Miller effect. For easy analysis, the capacitance C.sub.GD is treated on the assumption that it is so large as to be equivalent to the capacitance C.sub.GS and has a gate input capacitance which is a constant value C.sub.iss in parallel to the capacitance C.sub.GS. By virtue of such simplification, gate voltage increase of the power MOSFET 3 can be approximated by an exponential waveform having a time constant (.tau.=R.multidot.C.sub.iss) which is the product of a resistance value R of the gate resistor 110 and C.sub.iss of the power MOSFET 3.
In the power MOSFET 3, provided that a drain current i.sub.D starts to flow when the gate voltage v.sub.G exceeds V.sub.GS(OFF) and the drain current (or a load current) i.sub.D having a current value I.sub.D can flow when the gate voltage v.sub.G is equal to V.sub.GS(ON), the drain current i.sub.D can vary from O to I.sub.D while the gate voltage v.sub.G is increased from V.sub.GS(ON). If this period of time is defined as rise time t.sub.r, ##EQU1## is obtained. For example, if R=10.OMEGA., C.sub.iss =10 nF, V.sub.GG =10 V, V.sub.GS(OFF) =2V, and V.sub.GS(ON) =8V, ##EQU2## This charging characteristic is shown by the example of non-resonance in FIGS. 11(a) and (b). The gate voltage v.sub.G rises up to V.sub.GH which is substantially equal to the gate drive power source voltage V.sub.GG with the passage of time. Thereafter, when the control input V.sub.1 is switched to the low level, the gate input capacitance C.sub.iss of the MOSFET 3 which has been charged is discharged through the gate resistor 110 and the switching transistor 102, and the gate voltage v.sub.G is decreased. The decrease characteristic at this time is also indicated by the exponential function having the time constant .tau.=R.multidot.C.sub.iss and has fall time t.sub.f expressed by the same equation as Equation (1).
FIG. 12 is a circuit diagram showing a conventional flash controller using the gate drive circuit of FIG. 9. In FIG. 12, numeral 1 designates a high voltage power source (though, in practice, the voltage of a battery is often increased by a DC-DC converter), 2 designates a main capacitor charged by the high voltage power source 1, 6 designates an IGBT which is one type of the insulated gate semiconductor devices, 7 designates a flash discharge tube, 100 designates the gate drive circuit having the same constitution as FIG. 9 for on-and-off-driving the IGBT 6, and 200 designates a trigger circuit. The IGBT 6 and the flash discharge tube 7 are connected in series to each other, and this series connected body is connected to the main capacitor 2 in parallel. The trigger circuit 200 comprises a trigger transformer 8, a trigger capacitor 9 and a charging resistor 10. Trigger pulse energy is supplied to the trigger circuit 200 according to the variation of a collector voltage of the IGBT 6.
Referring now to a waveform chart of FIG. 13, an operation is explained hereinafter. The control input V.sub.1 is low. A voltage of about 30 V is applied to the gate drive power source V.sub.GG, and a voltage of about 300 V is applied to the high voltage power source 1. The gate voltage v.sub.G of the IGBT 6 is held at about the ground level because the switching transistor 101 is off and the switching transistor 102 is on. The collector voltage of the IGBT 6 is charged up to about 300 V as well as the trigger capacitor 9 by the charging resistor 10 in the trigger circuit 200.
In such a state, when the control input V.sub.1 is switched to a high level, the switching transistor 101 is turned on and the switching transistor 102 is turned off. Accordingly, the gate input capacitance C.sub.iss is charged by the gate drive power source V.sub.GG through the switching transistor 101 and the gate resistor 110, thereby the gate voltage v.sub.G of the IGBT 6 being increased. When the gate voltage v.sub.G is increased up to a value by which the IGBT 6 can be turned on enough, the collector voltage of the IGBT 6 is decreased rapidly, and the trigger capacitor 9 which has been charged is discharged through a primary winding of the trigger transformer 8 to the collector of the IGBT 6, thereby a high voltage pulse of several KV being generated at a secondary winding of the trigger transformer 8. The gate of the flash discharge tube 7 is thus triggered and the flash discharge tube 7 conducts. The main capacitor 2 which has been charged is discharged through the flash discharge tube 7 and the IGBT 6, and flash discharge starts.
Thereafter, when the flash amount required for photography etc. is obtained and then the control input V.sub.1 is switched to the low level again, the switching transistor 101 is turned off and the switching transistor 102 is turned on. Accordingly, the gate input capacitance C.sub.iss of the IGBT 6 which has been charged is discharged through the gate resistor 110 and the switching transistor 102. When the gate voltage v.sub.G of the IGBT 6 is decreased to be lower than the voltage by which the IGBT 6 can hold on-state, a collector current i.sub.c starts to be decreased. Thereafter the collector current i.sub.c of the IGBT 6 is decreased with the decreasing gate voltage v.sub.G, and finally the IGBT 6 is turned off. Thereby the flash emission of the flash discharge tube 7 is terminated. The desired amount of light required for photography etc. can be thus obtained by controlling the conducting period. For flash control by the IGBT 6, it is necessary to treat a large current of 100-200 A peak in a practical flash discharge tube. For the achievement in an economical device, the gate voltage must be considerably higher than that of other applications.
In the conventional gate drive circuit 100 of FIG. 9, since the gate input capacitance C.sub.iss of the power MOSFET 3 is charged and discharged by the gate resistor 110, the output voltage thereof varies merely exponentially between the ground and V.sub.GG. For high-speed drive of the power MOSFET 3, the resistance of the gate resistor 110 must be small. Accordingly the peak value (I.sub.GP1 .apprxeq.I.sub.GP2 =V.sub.GG /R) of the current which flows through the switching transistors 101 and 102 grows large. As a result, the switching transistors 101 and 102 with large current capacitances must be used, and the circuit is difficult to be integrated. In addition, switching losses of the switching transistors 101 and 102 increase, and driver losses are liable to increase. When a current for charging and discharging the gate input capacitance C.sub.iss of the power MOSFET 3 is conducted, the gate resistor 110 generates a power loss P.sub.d indicated by Equation (3). EQU P.sub.d =f.multidot.C.sub.iss .multidot.V.sub.GG.sup.2 ( 3)
For example, when f=1 MHz, C.sub.iss =10 nF and V.sub.GG =10 V, Pd=1 W is obtained, which is too large to be disregarded as driver losses.
On the other hand, in the conventional flash controller shown in FIG. 12, a high voltage (about 30 V in current devices) capable of sufficiently driving the IGBT 6 in on-state is required for the gate drive power source V.sub.GG. In general, a flash device for a camera has a 3-6 V power source of a battery and a high voltage power source of about 300 V obtained by increasing the voltage of the 3-6 V power source by means of the DC-DC converter, as power sources. However, the device does not have a power source of about 30 V suitable for driving the gate of the IGBT 6. Such a power source must be produced as a separate circuit. Many current AF cameras contain 12 V power sources for CCD driving. In such cases, it is possible to control a current of about 130 A by directly driving the IGBT by means of the 12 V power source if the IGBT with a low gate drive voltage is used.
Conventionally, the separate power source V.sub.GG of about 30 V has been normally produced by making a tap from an output winding of the DC-DC converter for generating a high voltage power source to rectify and smooth the voltage of the tap, or by dividing the 300 V voltage of the high voltage power source by the use of a switching element such as a high breakdown voltage transistor to produce a temporary power source.
A power circuit of FIG. 14 corresponds to the former, which is provided with a 3-6 V power source 301 of a battery, a DC-DC converter 300 comprising a transistor 302 and a step-up transformer 303, a smoothing circuit for a high voltage power source V.sub.CM comprising a diode 304 and the main capacitor 2, and a smoothing circuit for the gate drive power source V.sub.GG comprising a diode 305 and a capacitor 306. In a flash device in which V.sub.GG is simultaneously generated while the DC-DC converter 300 is in operation and the operation of the DC-DC converter 300 is terminated when the high voltage power source V.sub.CM reaches the specified output voltage (as is a current method for reducing battery consumption), charging of the gate drive power source V.sub.GG stops according to the cessation of oscillation of the DC-DC converter 300, and the consumed current of the gate drive circuit 100 and the leakage current of the capacitor 306 decrease the output voltage of the capacitor 306. In applications in which particularly large current is used such as flash control, because shortage of the gate voltage of the IGBT is fatal and causes element breakage, the decrease in the output voltage must be controlled to be minimum. Therefore, it is necessary to sufficiently increase the capacitance of the capacitor 306 or to sufficiently decrease the consumed current of the gate drive circuit 100. For example, for holding the gate drive power source V.sub.GG at 33 V to 28 V for a minute when the consumed current of the gate drive circuit 100 is 10 .mu.A, the required capacitance of the capacitor 306 is 120 .mu.F. A capacitor having such capacitance and a breakdown voltage of about 50 V is considerably large and costly so that the economical burden on the flash controller for which low cost is required is too large.
A power circuit of FIG. 15 corresponds to the latter. In the power circuit, transistors 406 and 402 are turned on by an emission start signal from a camera, and thereby a capacitor 400 which has been charged through a resistor 401 by the high voltage power source V.sub.CM is discharged through the high voltage transistor 402 and a resistor 403 to a constant voltage diode 404. A capacitor 405 is charged with this voltage, which is used for the gate drive power source V.sub.GG. In this power circuit, the transistor 101 and the logical circuit 103 of FIG. 12 can be omitted. This type of power circuit needs a high breakdown voltage transistor of more than 300 V and a large number of components, so that the economical burden on the flash controller for which low cost is strongly desired is also too large.
If an IGBT having the low gate drive voltage is developed and used as the IGBT 6 of FIG. 12, the lower limit value V.sub.GE(OFF) of the gate voltage v.sub.G (or a gate-emitter threshold voltage V.sub.GE(th)) for holding the IGBT 6 in off-state falls down to about one-third compared with the case of using the normal IGBT. Therefore, the gate voltage v.sub.G must be lower than the threshold voltage for turning-off drive. However, in the circuit of FIG. 12, since the gate voltage v.sub.G shows exponential decrease, the off-drive of the gate is slowed as the gate voltage v.sub.G is decreased. When the off-drive of the gate is too slow, turning-off of the IGBT 6 is also slowed and the amount of flash light grows larger than desired and expected, creating a possibility of overexposure in photography, particularly in close-up picture taking.